February 3, 2006
Measurements of 2003 UB313's thermal emission reveal the "tenth planet" to be bigger than Pluto.
"It's definitely bigger than Pluto."
So said Caltech astronomer Michael Brown during a July 29, 2005, press conference announcing his team's discovery of 2003 UB313, the tenth planet. Recent observations by astronomers at the University of Bonn and the Max Planck Institute for Radio Astronomy, both in Germany, prove Brown correct.
At the time, Brown made this claim based on the object's optical brightness. Knowing an object's size, however, is impossible without knowing its reflectivity. While Brown and his team estimated 2003 UB313's diameter at 1,780 miles (2,860 kilometers), Wilhelm Altenhoff's team concluded it is 1,860 miles (3,000 km) wide. Pluto measures 1,485 miles (2,390 km) across.
Altenhoff used the Max Planck Millimeter Bolometer detector on the Institut de Radio Astronomie MillimÓ?trique (IRAM) 30m telescope in Spain to measure the amount of heat 2003 UB313 radiates. The detector measures heat radiation at a wavelength of 1.2mm, which registers brightness only from the object's surface temperature and its size, not reflected sunlight. In 1988, Altenhoff measured Pluto's thermal emission using a similar device.
"Since UB313 is decidedly larger than Pluto, it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status," says Frank Bertoldi, leader of the University of Bonn team that measured the solar system object.